The FT-reaction for conversion of synthesis gas, a mixture of CO and hydrogen, possibly also containing essentially inert components like CO2, nitrogen and methane, is commercially operated over catalysts containing the active metals iron (Fe) or cobalt (Co). However, the iron catalysts exhibit a significant shift reaction, producing more hydrogen in addition to CO2 from CO and steam. Therefore, the iron catalyst will be most suited for synthesis gas with low H2/CO ratios (<1.2) e.g. from coal or other heavy hydrocarbon feedstock, where the ratio is considerably lower than the consumption ration of the FT reaction (2.0-2.1).
Normally, the active FT metal is dispersed on a solid, porous support. Thereby, a large portion of the Co is exposed as surface atoms where the reaction can take place. The support can be alumina, titania or silica, but generally speaking, other oxides like zirconia, magnesia, silica-alumina, various aluminates, zeolites as well as carbon, and mixtures thereof, have been used. Sometimes the support contains modifying components ingredients, e.g. of compounds of silicon, lanthanum, titanium and zirconium.
To enhance the catalyst performance, e.g. by facilitating reduction of cobalt oxide to cobalt metal prior to the FT synthesis, it is common to add different promoters, and rhenium, ruthenium, platinum, iridium and other transition metals can all be beneficial. Alternatively, the promoter may be alkali metals or alkaline earth metals. It has been discovered that certain amounts of alkali metals (K, Na, Cs, Rb, Li, Cs) have a significant impact on the catalytic performance of cobalt supported catalysts. U.S. Pat. No. 4,880,763 (Eri et al) reported that addition of an alkali to the catalyst serves to increase the average molecular weight of the product, as shown by an increase in the Schulz-Flory α value. However, the activity decreased as the alkali content increased. Thus, for any particular situation, there is an optimum alkali level that balances the desired average product molecular weight and catalyst activity. In WO2006/010936 A1, Rytter and Eri described the effect of Na on cobalt catalysts. A clear, negative effect on the activity was discovered in the range 0 to 500 ppm.
Luo and Davis (Fischer-Tropsch synthesis: Group II alkali-earth metal promoted catalysts, Appl. Catal. A 246 (2003) 171) compared the effect of alkaline earth metals, among them calcium, on the Fischer-Tropsch synthesis performance over an iron-based catalyst in a continuous stirred tank reactor (CSTR). They found that the addition of calcium as a promoter has a negative effect on the activities of both Fischer-Tropsch synthesis and the water-gas shift reaction. However, Ca generated a higher FTS alpha value (chain growth probability) than the un-promoted catalysts.
An FT catalyst is operated in an industrial process in which synthesis gas (syngas; a gas mixture of H2 and CO which possibly also can contain other gases e.g. CO2, CH4, light hydrocarbons, steam, N2 etc.) is converted to hydrocarbons by the FT-process. Syngas can be prepared in a number of ways such as autothermal reforming (ATR), methane steam reforming (MSR) sometimes called tubular reforming, partial oxidation (POx), catalytic partial oxidation (CPO) and gasification. The latter is primarily used for other feeds than natural gas, typically coal or biomass. Combinations and optimizations of these processes are also possible, as in combined reforming, heat exchanged reforming, compact reforming and gas heated reforming.
Following syngas generation, frequently the gas is cooled down in a waste heat boiler (WHB), also called process gas cooler, and further energy can be extracted from the gas by using a superheater to enhance the temperature in generated steam. Before the gas enters the FT-reactor, the gas may be cleaned of impurities like ammonia and sulphur and various carbonyl compounds using guard beds. Both in the syngas generation and in the cleaning process, refractory or ceramic materials are frequently employed. These can consist of mixtures of various metal oxides. It has now been discovered that great care must be taken as to the composition of these materials.
In Catalyst Handbook (Catalyst Handbook, 2nd edition, M. V. Twigg, editor, Wolfe Publishing, London 1989), poisoning by impurities is described on pages 77-81. However, the effect of calcium is not described and sodium only in relation to hydrocracking catalysts. Furthermore, carryover of materials from the syngas or gas cleaning sections is not described. It has now been discovered that great care must be taken to avoid such carryover. Carryover through the syngas can be enhanced by the presence of steam. Certain materials used in syngas generation have been described by R. Stevens and U. R. Desai, Qatar Fertilizer Company, in the proceedings of Nitrogen+Syngas 2008 conference conducted in Moscow, 20-23 Apr. 2008. These include alumina lumps used in the upper part of the secondary reformer (ATR) that contain 0.7 wt % NaO, use of sodium aluminate, NaAl11O18 and a ceramic lining (donuts) that contains 17 wt % CaO.